Photoelectrophoretic imaging process using dry pigment coated substrate

ABSTRACT

A dry particulate layer of electrically photosensitive particles is placed on a substrate. A liquid is then applied to the dry particulate layer. The resulting suspension is used in photoelectrophoretic imaging.

United States Patent [191 Stein et a1.

[ Dec. 31, 1974 PHOTOELECTROPHORETIC IMAGING PROCESS USING DRY PIGMENT COATED SUBSTRATE Inventors: Ira S. Stein; Leonard M. Carreira,

both of Penfield, N.Y.

Assignee: Xerox Corporation, Rochester, NY.

Filed: June 2, 1969 Appl. No.2 829,698

Related U.S. Application Data Continuation-impart of Ser. No. 510,637, Nov. 30, 1965, abandoned.

U.S. Cl 96/l.2, 96/1.3, 96/1 R,

117/37 LE, l17/93.4 A Int. Cl G03g 17/00 Field of Search 96/1.2, 1.3, 1.5, 1.6,

96/1.7; 204/181; 117/37 LE, 93.4 A

Primary Examiner-George F. Lesmes Assistant Examiner-Charles E. Lipsey Attorney, Agent, or Firm-James .l. Ralabate; David C. Petre; Richard A. Tomlin 5 7 ABSTRACT A dry particulate layer of electrically photosensitive particles is placed on a substrate. A liquid is then applied to the dry particulate layer. The resulting suspension is used in photoelectropho'retic imaging.

6 Claims, 2 Drawing Figures PATENTEDUEESI 1914 3 857 7 07 INVENTOR. IRA S. STEIN LEONARD M. CARREIRA Lac/aw ATTORNEY PHOTOELECTROPIIORETIC IMAGING PROCESS USING DRY PIGMENT COATED SUBSTRATE This application is a continuation-in-part of our copending application Ser. No. 510,637 filed Nov. 30, 1965 now abandoned.

BACKGROUND OF THE INVENTION This invention relates in general to imaging systems and, more specifically, to an improved photoelectrophoretic imaging system.

There has been recently developed a photoelectrophoretic imaging system capable of producing color images which utilizes electrically photosensitive particles. This process is described in detail and claimed in U.S. Pat. No. 3,384,565, US. Pat. No. 3,384,566, US. Pat. No. 3,383,993 and U.S. Pat. No. 3,384,488, all issued May 21, 1968. In such an imaging system colored light absorbing particles are suspended in a nonconductive liquid carrier. The suspension is placed between electrodes, one of which is preferably conductive and called the injecting electrode and one having an insulating layer on its surface and referred to as the blocking electrode, subjected to a potential difference and exposed to an image. As these steps are completed, selective particle migration takes place in image configuration, providing a visible image at one or both of the electrodes. An essential component of the system is the suspended particles which must be electrically photosensitive and which apparently undergo a net change in charge polarity upon exposure to activating electromagnetic radiation, through interaction with one of the electrodes. In a monochromatic system, a single colored image is produced equivalent to conventional black-and-white photography. In a polychromatic system, the images may be produced in natural color by providing a mixture of particles of two or more different colors which are each sensitive to light of a specific wave-length or narrow range of wave-lengths. Particles used in this system must have both intense and pure colors and be highly electrically photosensitive.

In the above-described process, the pigment particles are suspended in an insulating carrier liquid and the suspension is coated on one of the electrodes before the imaging process takes place.

Coating of a thin liquid layer of the desired thickness from the suspension is difficult. Care must be exercised that excessive evaporation of the carrier liquid does not take place between the coating step and the imaging step. Storage of the suspension before use is a problem since thepigments often tend to settle out of the suspension. Also, pigments of different colors may not settle out at equal rates requiring careful stirring or resuspension of the pigments immediately before the coating step. While, when carefully done, the process including the step of coating the suspension onto an electrode immediately before imaging is capable of producing images of excellent quality, the inclusion of this coating step in the process undesirably complicates the entire system.

SUMMARY OF THE INVENTION It is, therefore, an object of this invention to provide a photoelectrophoretic imaging system which overcomes the above noted disadvantages.

It is another object of this invention to provide a photoelectrophoretic imaging system which does not reand which may be activated by the application of a carrier liquid thereto immediately before or during the imaging process.

The pigment is preferably coated onto a support and allowed to dry thereon. The mixture is held to the support by a binder, such as wax, which is soluble in the insulating carrier liquid. The resulting sheet is capable of being handled or stored without degradation. The pigment mixture may be activated, that is, returned to liquid suspension form, by the application of a suitable insulating carrier liquid thereto immediately before or during the imaging step. The carrier liquid may be sprayed onto the surface of the sheet or applied by any other suitable method.

BRIEF DESCRIPTION OF THE DRAWINGS The advantages of this improved photoelectrophoretic system will become apparent upon consideration of the following detailed disclosure of the invention, especially when taken in conjunction with the accompanying drawings wherein:

FIG. 1 shows a side view of a simple exemplary system for carrying out the process of this invention wherein the dry pigment material carrying sheet is placed on one electrode and the carrier liquid is sprayed thereon immediately before imaging, and

FIG. 2 shows an alternative embodiment in which the pigment mixture coated sheet is placed on an electrode and carrier liquid is applied by rupturing a pod.

Referring now to FIG. 1, there is seen a transparent electrode generally designated 1 which, in this exem-.

plary instance, is made up of a layer of optically transparent glass 2 overcoated with a thin optically transparent layer 3 of tin oxide, commercially available under the name NESA glass. This electrode hereinafter is referred to as the injecting electrode." In close contact on the surface of injecting electrode 1 is an imaging sheet 4 which comprises a transparent, relatively conductive substrate 5 carrying a thin dry layer 6 of finely divided electrically photosensitive particles and a wax binder. This layer may have a thickness of from about 1 to about 20 microns. For optimumcolor density and balance, it is preferred that this layer have a thickness of from about 3 to about 10 microns. Where the imaging sheet is placed on the injecting electrode, and imagewise exposure is made through it, the substrate should be substantially transparent. Substrates with resistivities of up to 10" ohm-centimeters are preferred when placed on the injecting electrode since the lower resistivity allows more efficient charge exchange. Regenerated cellulose is a suitable material having these characteristics. The term photosensitive," for the purposes of this application, refers to the properties of a particle which, once suspended in an insulating carrier liquid can be attracted to an electrode, and will migrate away from it under the influence of applied electric field when it is exposed to electromagnetic radiation. For a detailed explanation of the apparent mechanism of operation of the imaging process, see the above mentioned patents. The wax binder which holds the pigment layer 6 to substrate may be any material easily soluble in the insulating carrier liquid to be used in the process.

Adjacent to injecting electrode 1 is a second electrode generally designated 7, hereinafter called the blocking electrode, which is connected to one side of the potential source 8. The opposite side of potential source 8 is connected to switch 9 and to the injecting electrode 1 so that when switch 9 is closed an electric field is applied across the imaging sheet 4 between electrodes 1 and 7. An image projector made up of a light source 10, a transparency and a lens 12 is provided to expose the suspended particles of layer 6 to a light image of the original transparency 11 to be reproduced. Electrode 7 is made in the form of a roller having a central conducting core 13 connected to the potential source 8. The core is covered with a layer of a blocking electrode material, 14, which may be Baryta paper. Spray nozzle 15 is provided to apply an insulating carrier liquid to the pigment layer 6 immediately before imaging. This liquid may be any insulating material which is a good solvent for the wax binder holding the particles in layer 6 to substrate 5. The carrier liquid is sprayed onto the surface of layer 6, dissolving the binder material and causing a suspension of the pigment particles.

The pigment suspension is exposed to the image to be reproduced and a potential is applied across the blocking and injecting electrodes by closing switch 9. Roller 7 is caused to roll across the top surface of injecting electrode 1 with switch 9 closed during the period of image exposure. This light exposure causes exposed pigment originally attracted to electrode 1 to migrate through the liquid and adhere to the surface of the blocking electrode, leaving behind a pigment image on the surface of imaging sheet 4 which is a duplicate of the original transparency 11. Particles adhering tothe surface of blocking electrode 7 may be cleaned therefrom and the exposure steps repeated, if desired. Additional exposure and cleaning steps have been found to increase color purity and color balance. After exposure, the relatively volatile carrier liquid evaporates off, leaving behind the pigment image. The original wax binder in layer 6 comes out of solution and serves to fix the image to substrate 5 and permit handling thereof. If desired, the image may be further made permanent by spraying a lacquer coating thereover or by laminating a sheet thereover.

In the embodiment generally shown in FIG. 1, the imaging sheet 4 could be applied to the surface of conductive roller 13, the substrate 5 forming the blocking surface instead of layer 7. Layer 7 may thus be deleted from the system. Spray nozzle 15 directs the insulating carrier liquid spray against the blocking electrode surface or against the surface of injecting electrode 1 during imaging. An image is formed directly on the NESA surface 3, and is transferred therefrom by suitable means, for example, that disclosed in copending application Ser. No. 459,860 filed May 28, 1965 now abandoned. Where imaging sheet 4 is applied to the blocking electrode surface, substrate 5 is preferably highly insulating. Where a separate blocking surface such as layer 7 is used layer 5 need not be of highly insulating material.

FIG. 2 shows a second embodiment of the pigment particle suspension means. The schematically shown exemplary structure is generally similar to that of FIG. 1. Similar elements are numbered as in FIG. I.

In the embodiment of FIG. 2, injecting electrode 1, blocking electrode 7 and electrical system 8, 9 are the same as that of FIG. 1 The imaging sheet 4 is exposed through the injecting electrode 1 by a conventional projection system (not shown) suchas that shown in FIG. 1. The means for supplying insulating carrier liquid to the dry pigment mixture 6 coated on the transparent, conductive layer 5 differs from that of FIG. 1. A capsule 17 extending across the edge of sheet 5 in a direction parallel to the axis of roller blocking electrode 7 contains the insulating carrier liquid 16. Preferably, the wall of capsule 17 is weakened in a direction such that when pressure is applied to the capsule it will rupture and the contained carrier liquid will be applied to pigment layer 6. As shown in FIG. 2, when roller 7 approaches imaging sheet 4 it first contacts capsule 17. Capsule 17 ruptures and roller 7 spreads the carrier liquid across pigment layer 6. Although the carrier liquid contacts the pigment mixture only shortly before or during the imaging step, images of good quality with good color separation result. Capsule 17 may comprise any material which is substantially impermeable to the encapsulated insulating carrier liquid 16. Imaging sheet 4 carrying capsule 17 may be easily stored for extended periods, then placed on the surface of injecting electrode 1, or conductive roller 13 and an image conveniently made. The imaging member substrate may comprise any suitable material. Where transparent conductive substrates are used any suitable material may be used. Typical transparant conductive materials include conductively coated glass such as tin oxide coated glass or transparent metallic coatings on transparent plastics or relatively conductive transparent plastics such as regenerated cellulose. Where an insulating material is used as the imaging member substrate any suitable insulating material may be used. Typical insulating materials include: insulating rubber, baryta paper, cellulose acetate, polyethylene coated paper, nitro cellulose,

polystyrene, I polytetrafluoro ethylene, polyvinylfluoride, polyethylene terephthalate and mixtures thereof.

Where insulating substrates are used DuPont Tedlar polyvinyl fluoride film is preferred because it combines high dielectric constant with high dielectric strength and is easy to handle. Where conductive transparent substrates are used regenerated cellulose is preferred because of its conductivity, transparency and ease of handling.

To increase the ease of handling and storage of the dry pigment mix coated substrates, a very small proportion of an easily soluble wax should preferably be in-.

cluded in the pigment mix. About l-6% by weight of paraffin wax in the pigment mix has been found to produce good results. Any suitable easily soluble wax or other material may be used. Typical materials include paraffimic and microcrystalline petroleum waxes such as Sunoco Wax 5825, and Halocarbon Wax 600.

The insulating carrier liquid applied to the dry pigment layer may comprise any suitable material. The material should be a good solvent for the temporary binder used in the dry pigment mixture layer, be reasonably volatile and be a good insulator. Excellent results have been obtained with Sohio Odorless Solvent 3440 (a mixture of kerosene fractions) and lsopar G (a long chain saturated aliphatic hydrocarbon) therefore these are preferred materials. Any other suitable insulating carrier liquid may be used. Typical materials are decane, dodecane, N-tetradecane, silicone oils, petroleum ethers, paraffin oils and mixtures thereof.

Where the insulating carrier'liquid is to be held in a rupturable capsule prior to application to the dry pigment layer, an insulating material should be chosen which is impermeable to the encapsulated liquid and which may be easily ruptured during the imaging operation. The selection of an encapsulating material will be determined by the characteristics of the encapsulated liquid. Typical encapsulating materials include polyvinylchloride, polyvinylfluoride, polyvinylacetate, polystyrene, vinylchloridevinylacetate copolymers and mixtures thereof.

The following examples further specifically define the present invention with respect to the use of dry pigment particle layers in photoelectrophoretic imaging systems. Parts and percentages are by weight unless otherwise indicated. The examples below are intended to illustrate various preferred embodiments of the photoelectrophoretic imaging system of this invention. Each of the following examples is carried out in an apparatus of the general type illustrated in FIG. I. In different examples, however, the dry pigment layer may be placed directly on the NESA glass substrate, or on the blocking electrode surface, or on a transparent substrate placed over the NESA glass substrate or on a conductive roller. The NESA substrate is connected in series with a potential source, a switch and the conductive center of a roller having a coating of Baryta paper on its surface. The roller is approximately 2% inches in diameter and is moved across the plate surface at about 1.5 centimeters per second. The plate employed is about 3 inches square and is exposed with a light intensity of about 8,000 foot candles as measured on the uncoated NESA glass surface. Unless otherwise indicated, 7% by weight of the indicated pigments in each'example are suspended in Sohio Odorless Solvent 3440, a mixture of kerosene fractions and the magnitude of the applied potential is about 2,500 volts. Exposure is made with a 3,200 K. ,lamp through a multicolor Kodachrome" transparency.

EXAMPLE I A tri-mix comprising a cyan-pigment, Monolight Fast Blue GS, the alpha form of metal-free phthalocyanine, C. I. No. 74100, a magenta pigment, Watchung Red B, l-(4'-methyl-5 -chloroazo benzene-2 '-sulfonic acid)-2- hydroxy-3-naphthoic acid, C. I. No. 15,865, available from E. l. du Pont de Nemours, and a yellow pigment, Algol Yellow -GC, 1,2,5,6-di(C,C'-diphenyl)- thiazoleanthraquinone, C. I. No. 67,300, available from General Dye Stuffs, is dispersed in about 100 parts Sohio Odorless Solvent 3440. The suspension is coated onto the NESA glass surface and allowed to dry. Additional Sohio Odorless Solvent 3440 is then sprayed onto the dry pigment mixture surface and immediately thereafter a potential of 2,500 volts is imposed between the injecting and blocking electrode as the blocking electrode roller passes across the pigment layer surface. Upon completion of the imaging step a positive image conforming to the original of excellent quality is seen on the NESA surface. The particulate image on the NESA surface may be transferred to a receiving sheet by any conventional means, such as that described in copending application Ser. No. 459,860, filed May 28, 1965 now abandoned.

EXAMPLE II A tri-mix is prepared as in Example l. To the liquid suspension is added about 3% by weight of a paraffin wax. The mixture is coated onto a 10 micron sheet of regenerated cellulose and allowed to dry. This sheet is then easily handleable without damage to the dry pigment layer. The surface of the injecting electrode is moistened with tap water to insure good electrical contact and the uncoated surface of the regenerated cellulose sheet is placed there against. Sohio Odorless Solvent 3440 is sprayed onto the dry pigment surface and the imaging steps are performed as described above. Upon completion of the imaging steps, a positive image conforming to the original of good quality is seen on the regenerated cellulose surface. Residual carrier liquid is allowed to evaporate, leaving an image on the regenerated cellulose layer which can be handled without damage.

EXAMPLE III A tri-mix is prepared as in Example 1 above. To the suspension is added about 2% paraffin wax. This suspension is coated onto a sheet of Baryta paper and allowed to dry. The Baryta paper is then wrapped around the blocking electrode roller and secured in place. This surface is sprayed with Sohio Odorless Solvent 3440 and the above described imaging steps are repeated. An image of good quality conforming to the original is observed on the NESA glass surface upon completion of the imaging steps. Residual pigment remainson the Baryta paper surface.

EXAMPLE IV The experiment of Example III is repeated except that the tri-mix is coated on Tedlar. The Tedlar sheet is wrapped around the conductive blocking electrode roller so that the Tedlar acts as the insulating blocking electrode surface as well as the tri-mix substrate. An image of good quality conforming to the original is observed on the NESA glass surface upon completion of the imaging steps. A negative image adheres to the Tedlar surface. This image may also be used where desired and is particularly useful for monochrome images.

Although specific components and proportions have been stated in the above description of the invention, other similar materials, as listed above, may be used with similar results.

In addition, other materials may be added to the dry pigment mixture layer, transparent conductive substrate, insulating carrier liquid, etc. to synergize, enhance, or otherwise modify their properties. For example, electrical sensitizers may be added to the dry pigment mixture or conductivity enhancing agents may be added to the transparent substrate material.

Other modifications of the present invention will occur to those skilled in the art upon reading the disclosure. These are intended to be included within the scope of this invention.

What is claimed is:

l. The method of photoelectrophoretic imaging comprising the steps of a. providing two electrodes at least one of which is at least partially transparent and providing a dry imaging layer comprising electrically photosensitive particles dispersed in a soluble binder material on the surface of one of said electrodes;

b. applying an insulating carrier liquid to said dry imaging layer to form a liquid imaging suspension layer immediately before or during performing steps and (d) recited below;

0. subjecting said suspension layer to an applied electrical field between said electrodes; and

d. exposing said suspension layer to imagewise activating electromagnetic radiation through said transparent electrode until an image is formed.

2. The method as defined in claim 1 wherein said dry imaging layer comprises pigments of one color dispersed in a soluble binder material and a monochromatic image is formed.

3. The method as defined in claim 1 wherein said dry imaging layer comprises-pigments of at least two different colors dispersed in a soluble binder material and a polychromatic image is formed.

4. The method of photoelectrophoretic imaging com- 8 prising the steps of a. providing a dry imaging layer comprising electrically photosensitive particles dispersed in a soluble binder material on a substrate;

b. applying an insulating carrier liquid to said dry imaging layer to form a liquid imaging suspension layer immediately before or during performing steps (c) and (d) recited below;

c. subjecting said suspension layer to an applied electrical field between two electrodes, at least one of which is at least partially transparent; and

d. exposing said suspension layer to imagewise activating electromagnetic radiation through said transparent electrode until an image is formed.

5. The method as defined in claim 4 wherein said dry imaging layer comprises pigments of one color dispersed in a soluble binder material and a monochromatic image is formed.

6. The method as defined in claim 4 wherein said dry imaging layer comprises pigments of at least two'different colors dispersed in a soluble binder material and a polychromatic image is formed. 

1. The method of photoelectrophoretic imaging comprising the steps of a. providing two electrodes at least one of which is at least partially transparent and providing a dry imaging layer comprising electrically photosensitive particles dispersed in a soluble binder material on the surface of one of said electrodes; b. applying an insulating carrier liquid to said dry imaging layer to form a liquid imaging suspension layer immediately before or during performing steps (c) and (d) recited below; c. subjecting said suspension layer to an applied electrical field between said electrodes; and d. exposing said suspension layer to imagewise activating electromagnetic radiation through said transparent electrode until an image is formed.
 2. The method as defined in claim 1 wherein said dry imaging layer comprises pigments of one color dispersed in a soluble binder material and a monochromatic image is formed.
 3. The method as defined in claim 1 wherein said dry imaging layer comprises pigments of at least two different colors dispersed in a soluble binder material and a polychromatic image is formed.
 4. THE METHOD OF PHOTOELECTROPHORETIC IMAGING COMPRISING THE STEPS OF A. PROVIDING A DRY IMAGING LAYER COMPRISING ELECTRICALLY PHOTOSENSITIVE PARTICLES DISPERSED IN A SOLUBLE BINDER MATERIAL ON A SUBSTRATE; B. APPLYING AN INSULATING CARRIER LIQUID TO SAID DRY IMAGING LAYER TO FORM A LIQUID IMAGING SUSPENSION LAYER IMMEDIATELY BEFORE OR DURING PERFORMING STEPS (C) AND (D) RECITED BELOW; C. SUBJECTING SAID SUSPENSION LAYER TO AN APPLIED ELECTRICAL FIELD BETWEEN TWO ELECTRODES, AT LEASR ONE OF WHICH IS AT LEAST PARTIALLY TRANSPARENT; AND D. EXPOSING SAID SUSPENSION LAYER TO IMAGEWISE ACTIVATING ELECTROMAGNETIC RADIATION THROUGH SAID TRANSPARENT ELECTRODE UNTIL AN IMAGE IS FORMED.
 5. THE METHOD AS DEFINED IN CLAIM 4 WHEREIN SAID DRY IMAGING LAYER COMPRISES PIGMENTS OF ONE COLOR DISPERSED IN A SOLUBLE BINDER MATERIAL AND A NONOCHROMATIC IMAGE IS FORMED.
 6. The method as defined in claim 4 wherein said dry imaging layer comprises pigments of at least two different colors dispersed in a soluble binder material and a polychromatic image is formed. 